This invention relates generally to a borescope or endoscope for providing a full color video image of an inaccessible area, and more particularly to an improved system for measuring the size of an object viewed by the imager head of the device for display on the video image display of the borescope or endoscope (hereinafter termed, collectively, "borescope").
The prior art shows a number of devices for providing a full color video picture of a remotely situated target, such as the inside of a jet engine or a human ulcer. Such devices have become increasingly sophisticated over time; they now include a video screen, an external light source, and an elongated flexible insertion tube. Video processing functions are usually handled outside the insertion tube. The insertion tube contains, through much of its length, fiber optic bundles for transmitting light from the external source to the imager head at distal end of the tube. This imager head comprises a solid state sensor and lens system set in a housing having a transparent window, usually at the tip, for viewing the target. The information from the solid state sensor is relayed to the external video display into which the proximal end of the insertion tube is connected or plugged.
When viewing a target through a borescope, there is often no reference standard which can be used to determine the size of the target, the observed size being a function not only of the target's size, but also of its distance from the imager head. U.S. Pat. No. 4,980,763 to Lia and owned by a common assignee of the present applicants, teaches a method for determining the size of a target viewed through a borescope. The method involves generating a shadow of a known shape in association with the illumination that is cast upon the target. The shadow appears in the plane of the target and both the target and the shadow are detected by the imager head. Because the location of the shadow generating image is known, the magnification of the image is also known, and therefore computation of the actual size of the target can be made. This information is relayed to the video screen display.
The borescope used in the earlier invention of Lia operates well to provide desired target information. However, because the illumination and shadow producing components lie on a parallel axis to that of the image detection system, the distal end of the insertion tube must be of sufficient diameter to accommodate these components while providing an area of the optically transparent tip large enough to allow substantial overlap of the areas of illumination and detection. In essence, the minimal diameter of this borescope is determined by the sum of the maximum width of the image detection system and the diameter of the cross section of the cone of light created by the illumination source in the plane of the optically transparent window at the tip of the borescope.
The U.S. Pat. No. 4,867,138 to Kubota et al. teaches a method wherein the outside diameter of the base side of a rigid electronic endoscope is made smaller than the tip part. The method involves the use of a prism to direct the image detection means at an angle of less than 90.degree. to the longitudinal axis of the housing of the endoscope. This method operates well to reduce the diameter of the base side of the rigid endoscope allowing a treating tool and irrigating liquid to be fed and drained. However, it does not reduce the diameter of the imager head because the image detection means is placed perpendicular to the longitudinal axis.
There are applications which require an insertion tube of smaller diameter than is possible using this configuration. A number of patents have been directed toward achieving a high degree of compactness. See, for an example U.S. Pat. No. 4,491,865 to Danna et al, owned by a common assignee of the present applicants. Thus, it is desirable to implement the shadow technique of Lia so that size measurements can be obtained, while creating a more compact insertion tube end than is shown in that disclosure.